Low-temperature etching environment

ABSTRACT

A low-temperature etching environment comprising a halogen and an inert gas in a ratio that does not induce the formation of an etch-limiting surface reaction layer during etching in the low-temperature etching environment. The surface temperature of a material being etched in the low-temperature environment is below that which would melt a photoresist material that has not been treated to increase its glass-reflow temperature.

TECHNICAL FIELD

[0001] Embodiments of the present invention relate to the field offorming microelectronic devices. Specifically, embodiments of thepresent invention relate to a low-temperature etching environmentcomprising a halogen and an inert gas.

Background Art

[0002] Conventional high density plasma etching of materials such asindium comprising compounds is subject to a tradeoff between etch rateand compatibility with standard photoresist masks and process steps. Itis possible to achieve a high etch rate if the surface temperature ofthe material being etched is high enough. Unfortunately, to achieve ahigh etch rate with conventional techniques, the surface temperature ofthe material being etched must be greater than the glass-reflow point ofstandard photoresist materials. The glass-reflow point of standardphotoresist material is about 110-120 degrees Celsius. The hightemperature can be achieved by either raising the temperature of thesubstrate of the material being etched or by allowing the high-densityplasma source to heat the surface of the material being etched.Consequently and undesirably, non-standard photoresists and/or processsteps must be used when etching in the high temperatures conventionallyneeded for a high etch rate.

[0003]FIG. 1 is a graph 100 of a surface temperature versus time curve110 and a etch rate versus time curve 120. Graph 100 illustrates theeffect that surface temperature has on etch rate in one conventionaletching system. FIG. 1 shows the surface temperature dependent etch ratefor indium phosphide (InP) when 100 watts (W) of microwave power wereused. The conventional etch environment corresponding to FIG. 1 furtherincludes a chlorine/argon (Cl₂/Ar) etchant with a flow rate of tenstandard cubic centimeters per minute (sccm) for both the chlorine andthe argon. Thus, the chlorine flow rate is 50 percent the combined flowrate of chlorine and argon. The pressure in the etch environment was0.27 pascals (Pa).

[0004] Referring again to FIG. 1, the surface temperature versus timecurve 110 shows that the surface temperature rose significantly duringthe etching and after about four minutes approached 150 degrees Celsius.The etch rate is about 100 nanometers per minute (nm/min) when thesurface temperature is below 100 degrees Celsius. Etch rates as high as2.4 micrometers per minute (μm/min) are shown for higher surfacetemperatures. However, the surface temperature must be close to 150degrees Celsius for the etch rate to be significantly greater than 100nm/min. More precisely, the etch rate is approximately 200 nm/min whenthe temperature is approximately 140 degrees Celsius, and the etch rateis approximately 450 nm/min when the temperature 150 degrees Celsius.Thus, in this conventional etch process, the surface temperature of thematerial being etched must be well above the glass-reflow point ofstandard photoresist materials to achieve a high etch rate.

[0005] While etching at a higher surface temperature achieves a higheretch rate, such higher surface temperatures are incompatible withstandard photoresist masks and processes and render the photoresist maskdifficult to remove. Hence, to etch in a high temperature environment, ahard mask such as silicon dioxide (SiO₂) or a silicon-nitrogen-hydrogencompound (SiNH_(x)) is conventionally used. Alternatively, a standardphotoresist mask having a low glass-reflow temperature can bepre-processed such that it will not melt at the higher surfacetemperature that is conventionally needed for a high etch rate.

[0006] When migrating to a new technology that etches a new material itis desirable to continue to use the same process steps that were usedwhen etching a material for a previous technology. However, using a hardmask or pre-processing a standard mask is incompatible with the processsteps of the previous technology. Moreover, removing a hard mask is moredifficult than removing a standard photoresist mask and the changes to astandard photoresist due to the pre-processing cause the photoresistmask to be difficult to remove after etching.

[0007] Thus, one problem with conventional etching methods is that toachieve a high etch rate the surface temperature of the material beingetched must be undesirably high. A further problem is the difficultyrealized in incorporating the photoresist into original processing stepswhen migrating to a new material being etched. A still further problemis the difficulty in removing a pre-processed photoresist once it hasbeen processed to withstand the high temperature conventionally neededfor a high etch rate. Alternatively, the surface temperature of thematerial being etched can be kept below the glass-reflow point of thephotoresist mask; however, conventional low-temperature etching methodshave a very low etch rate.

DISCLOSURE OF THE INVENTION

[0008] The present invention pertains to a low-temperature etchingenvironment. An embodiment in accordance with the present inventionprovides a low-temperature etching environment comprising a halogen andan inert gas in a ratio that does not induce the formation of anetch-limiting surface reaction layer during etching in thelow-temperature etching environment. The surface temperature of amaterial being etched in the low-temperature environment is below thatwhich would melt a photoresist material that has not been treated toincrease its glass-reflow temperature.

[0009] Another embodiment in accordance with the present invention is amethod of etching a surface reaction layer limited material. The methodcomprises receiving the surface reaction layer limited material in alow-temperature etching environment that comprises a halogen and aninert gas in a ratio that does not induce the formation of anetch-limiting surface reaction layer during etching in thelow-temperature etching environment. The method also comprises etchingthe surface reaction layer limited material within the low-temperatureetching environment.

[0010] Various embodiments in accordance with the present inventionachieve a high etch rate with a low surface temperature of the materialbeing etched. Embodiments do not require inconvenient pre-processingsteps to increase the glass-reflow temperature of the photoresist.Embodiments do not require the use of a process-incompatible hardphotoresist mask. Embodiments allow the use of a standard photoresistwith a low glass-reflow temperature. Thus, embodiments provide for easyremoval of the photoresist mask.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] The accompanying drawings, which are incorporated in and form apart of this specification, illustrate embodiments of the invention and,together with the description, serve to explain the principles of theinvention:

[0012]FIG. 1 is graph illustrating the effect of temperature on the rateof etching an indium-comprising material.

[0013]FIG. 2 is an exemplary device for producing a low-temperatureetching environment in accordance with embodiments of the presentinvention.

[0014]FIG. 3A is a graph illustrating etch rates achieved in accordancewith embodiments of the present invention.

[0015]FIG. 3B is a graph illustrating etch rates achieved in accordancewith embodiments of the present invention.

[0016]FIG. 4A is a graph illustrating resist selectivity achieved inaccordance with embodiments of the present invention.

[0017]FIG. 4B is a graph illustrating resist selectivity achieved inaccordance with embodiments of the present invention.

[0018]FIG. 5 is a flowchart illustrating a process of etching a surfacereaction layer limited material, according to an embodiment of thepresent invention.

[0019]FIG. 6 is a flowchart illustrating a process of inhibitingformation of a surface reaction layer in a low-temperature etchingenvironment, according to an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0020] During etching of some materials, a surface reaction layer formson the surface of the material being etched. The surface reaction layerhas a low volatility at low temperatures such that it accumulates to athickness that limits the etch rate. For purposes of the presentinvention, the term “low-temperature” means a temperature below whichwould melt a standard photoresist mask that has not been speciallytreated to withstand higher temperatures. For example, standardphotoresist material has a glass-reflow point of about 110-120 degreesCelsius. The surface reaction layer is believed to accumulate to athickness typically on the order of nanometers, which is sufficient tolimit the etch rate.

[0021] For example, when etching a material comprising InP, the low etchrate at low temperatures is believed to be due to the formation of asurface reaction layer of a compound of an indium and chlorine(InCl_(x)) layer on the InP surface. Conventionally, to achieve a highetch rate, the surface of the material being etched is raised to a hightemperature. The higher etch rates at higher temperatures are believedto be due to the increased volatility of InCl_(x) at the highertemperatures. Hence, higher temperatures reduce the thickness of thesurface reaction layer. This surface reaction layer is also referred toin the art as a selvedge layer.

[0022] The temperature to which the surface is heated to achieve a highetch rate will melt a photoresist material that has not been treated toincrease its glass-reflow temperature. Embodiments in accordance withthe present invention inhibit the formation of the surface reactionlayer by providing a halogen and an inert gas at a ratio that inhibitsthe formation of the surface reaction layer. As a result, the etch rateis higher than that obtained using conventional etching techniques in alow-temperature environment.

[0023] Embodiments according to the present invention are suited toetching any material whose etching is limited by the formation of asurface reaction layer. For purposes of the present application, theterm “surface reaction layer limited material” includes any material forwhich etching is adversely affected by the formation of a surfacereaction layer. InP is used herein as an example of one such surfacereaction layer limited material. However, the present invention is notlimited solely to the etching of indium-phosphide (InP). Furthermore,chlorine is used as an example of one etchant that contributes to theformation of the surface reaction layer. However, the present inventionis not limited to chlorine being the contributor to the surface reactionlayer.

[0024] Embodiments according to the present invention provide alow-temperature etching environment comprising a halogen and an inertgas. The etching environment comprising the halogen and the inert gasinhibits the formation of an etch-limiting surface reaction layer duringetching in the low-temperature etching environment. FIG. 2 is a sidesectional view of an exemplary device 200 for creating such alow-temperature etching environment. The exemplary device 200 is aninductively-coupled plasma reactor. However, the present invention isnot limited to inductively-coupled plasma reactors. The exemplary device200 comprises a power supply 202 coupled to an induction coil 204. Alsoincluded is a dielectric window 205. The exemplary device 200 also has awafer chuck 208 coupled to a bias power supply 210. Furthermore, a wafer212 is shown on the wafer chuck 208. The exemplary device 200 generatesthe low-temperature etching environment comprising a halogen and aninert gas (halogen and inert gas not depicted), which is introduced intoand passed through the etching environment. The present invention iswell suited to other plasma etch environments than theinductively-coupled plasma environment of FIG. 2. Such systems require asufficiently high plasma density to physically minimize the surfacereaction layer as it is being formed.

[0025] Embodiments according to the present invention introduce ahalogen and an inert gas into the etching environment in a ratio thatinhibits formation of an etch-limiting surface reaction layer. The inertgas serves as a diluent that reduces the concentration of the halogen.FIG. 3A is a graph 300 illustrating etch rate achieved in accordancewith embodiments of the present invention. Results are shown for etchingfour different materials, each comprising indium. The graph 300 shows anInP etch rate curve 310, an indium-gallium-arsenide (InGaAs) etch ratecurve 320, an indium-aluminum-arsenide (InAlAs) etch rate curve 330, andan aluminum-indium-gallium-arsenide (AlInGaAs) etch rate curve 340. Theresults show that the etch rate for each material is a function of theconcentration of the halogen to the inert gas. In this example, chlorineand argon were used for the halogen and the inert gas, respectively. Theetch rates were achieved under the following conditions. The processpressure was 0.27 Pa; the inductively-coupled power (ICP) was 1000 W;the reactive ion etching (RIE) power, which is also known as wafer chuckpower, was 55 W; the chuck temperature was −5 Celsius; the total gasflow was 15 sccm; and the etch time was three minutes.

[0026] Each curve (310, 320, 330, 340) in FIG. 3A shows an initialincrease in etch rate when the chlorine concentration increases abovezero percent. The etch rate for each curve peaks when the chlorineconcentration is below 20 percent. The chlorine concentration is definedas the flow rate of chlorine divided by the combined flow rate ofchlorine and argon. The data points of the various curves between about20 percent and 40 percent chlorine concentration show that the etch ratedecreases for each material as the chlorine concentration is furtherincreased. The data points of the various curves between 60-80 percentchlorine concentration show a slight increase in etch rate compared to achlorine concentration of about 40 percent. However, the etch rates forthe various curves at this chlorine concentration are well below thepeak etch rates which occur at a chlorine concentration of less than 20percent.

[0027] Conventionally, a high chlorine concentration is used based onthe belief that a higher chlorine concentration will provide a higheretch rate. This belief is based on analysis of the portion of the curveswith greater than 40 percent chlorine concentration without an awarenessof the portion of the curves with a chlorine concentration below 20percent. Moreover, using a low chlorine concentration to achieve a highetch rate is considered counterintuitive. In fact, a high chlorineconcentration results in excessive buildup of a surface reaction layerthat inhibits the etch rate. The present invention uses a low chlorineconcentration, which inhibits the formation of a surface reaction layer.

[0028] The surface reaction layer, if it exists at all, does not limitthe etch rate when the chlorine concentration is low. For example,referring to the data points in FIG. 3A in which the chlorineconcentration is less the 20 percent, the etch rate improves as thechlorine concentration increases. This indicates that if increasing thechlorine concentration forms a surface reaction layer, the negativeimpact of such increase on the etch rate is less than the positiveimpact on etch rate of increasing concentration of chlorine. Thus, thisis not an example of an etch-limiting surface reaction layer. However,at a higher chlorine concentration, the surface reaction layer limitsthe etch rate. For example, the etch rate of InP is about 275 nm/minwhen the chlorine concentration is about 10 percent. However, the InPetch rate is only about 110 nm/min when the chlorine concentration is atabout 35 percent. Similar results are indicated in FIG. 3A for the othermaterials. It is believed that with the higher chlorine concentration,the negative impact on etch rate of the surface reaction layer isgreater than the positive impact on etch rate of increased chlorineconcentration. For purposes of the present application, an“etch-limiting surface reaction layer,” is a surface reaction layer thatlimits the etch rate. Embodiments according to the present inventionprovide a low-temperature etching environment in which the halogen andthe inert gas are in a ratio that does not induce the formation of anetch-limiting surface reaction layer during etching.

[0029]FIG. 3A also shows that the selectivity between the variousmaterials being etched is a function of the chlorine concentration inthe etching environment. For example, the ratio of the InP etch rate tothe indium aluminum arsenide (InAlAs) etch rate is greater than one atlow chlorine concentrations. At a chlorine concentration of about sevenpercent, the etch rate of InP is about 280 nm/min, whereas the etch rateof InAlAs is about 220 nm/min. Thus, the InP/InAlAs etch rate ratio isabout 1.3 at about a seven percent chlorine concentration. However, athigher chlorine concentration, the etch ratio favors InAlAs. Forexample, at a chlorine concentration of about 68 percent, the etch rateof InP is about 120 nm/min, whereas the etch rate for InAlAs is about180 nm/min. This provides a selectivity of InP to InAlAs of only about0.7. The favorable etch rate of InP to InAlAs at a low chlorineconcentration, achieved in the present invention, is desirable anddifficult or impossible to achieve with conventional methods. Thefavorable etch ratio of InP/InAlAs at low temperature makes it easier tostop the etch at the InAlAs layer when etching a stack comprising InP ontop of InAlAs. Thus, embodiments according to the present invention areable to achieve a favorable etching selectivity between variousindium-comprising compounds. Such a favorable etching selectivity is notlimited to indium comprising compounds.

[0030]FIG. 3B shows a graph 350 illustrating etch rate achieved inaccordance with embodiments of the present invention using an electroncyclotron resonance (ECR) system. The results are similar to theinductively-coupled plasma (ICP) results shown in FIG. 3A. Results inFIG. 3B are shown for etching two different materials, each comprisingindium. The graph 350 shows an InP etch rate curve 360 and an InAlAsetch rate curve 370. The etch rate of InP is about 375 nm/min at achlorine concentration of about seven percent. The etch rates dropsbelow 200 nm/min when the chlorine concentration is about 40 percent.The etch rate of InAlAs is about 440 nm/min at a chlorine concentrationof about seven percent. The etch rates drops below 300 nm/min when thechlorine concentration is about 40 percent. Thus, it is believed that abuildup of an etch-limiting surface reaction layer limits the etch rateat higher chlorine concentrations.

[0031] Enhanced photoresist mask selectivity is another benefit ofembodiments according to the present invention. FIG. 4A shows a graph400 of selectivity ratio between the material being etched and thephotoresist mask under the same conditions as the data shown in FIG. 3A.FIG. 4A shows an InP/photoresist selectivity curve 410, anInGaAs/photoresist selectivity curve 420, an InAlAs/photoresistselectivity curve 430, and an AlInGaAs/photoresist selectivity curve440. Referring to FIG. 4A, the resist selectivity for all curves 410,420, 430, 440 is high at low chlorine concentrations relative to highchlorine concentrations. For example, when the chlorine concentration isbelow 20 percent, the resist selectivity is between two and eight forthe various materials being etched. Furthermore, the selectivity ratioof InP/photoresist peaks at greater than five and the selectivity ratiofor the other materials to photoresist is greater than four over most ofthe range below 20 percent chlorine. When the chlorine concentration isat about 40 percent or higher, the resist selectivity is much lower thanthe resist selectivity at a low chlorine concentration. Thus,embodiments according to the present invention provide a high etchselectivity between the material being etched and a photoresist maskwhen using a low-temperature etching environment in which the halogenand the inert gas are in a ratio that does not induce the formation ofan etch-limiting surface reaction layer during etching.

[0032] The increased etch selectivity of a semiconductor with respect tothe photoresist is because the lower concentration of chlorine reducesthe etch rate of the photoresist, as well as increasing the etch rate ofthe semiconductor as seen in FIG. 3A. The increased etch selectivity ofthe semiconductor with respect to the photoresist allows thinnerphotoresist masks to be used. Moreover, the increased etch selectivityfacilitates incorporating into standard process flows the fabrication ofdevices that are otherwise difficult to fabricate using standard processsteps. For example, heterojunction bipolar transistors are difficult tofabricate using standard process steps due to the formation of thesurface reaction layer. However, by utilizing embodiments in accordancewith the present invention, heterojunction bipolar transistors can befabricated using standard process methods.

[0033]FIG. 4B shows a graph 450 of the selectivity ratio between thematerial being etched and the photoresist mask for the ECR under thesame conditions as shown in FIG. 3B. The results are similar to the ICPresults shown in FIG. 4A. FIG. 4B shows an InP/photoresist selectivitycurve 460 and an InAlAs/photoresist selectivity curve 470. Referring toFIG. 4B, the resist selectivity for curves 460 and 470 is high at lowchlorine concentrations relative to the resist selectivity at higherchlorine concentrations. For example, when the chlorine concentration isbelow 10 percent, the resist selectivity for InP is 2.5 or greater. ForInAlAs, the resist selectivity is about four or higher for chlorineconcentrations below 10 percent. When the chlorine concentration is atabout 40 percent, the resist selectivity is below one for InP and aboutone for InAlAs.

[0034] An embodiment of the present invention is a method of etching amaterial for which etching is limited by a surface reaction layer.Referring to process 500 of FIG. 5, block 510 comprises receiving, in alow-temperature etching environment, a material for which etching in alow-temperature etching environment is limited by a surface reactionlayer. The low-temperature etching environment comprises a halogen andan inert gas. The halogen and the inert gas are provided in a ratio thatdoes not induce the formation of an etch-limiting surface reaction layerduring etching in the low-temperature etching environment.

[0035] In block 520, the process comprises etching the surface reactionlayer limited material within the low-temperature etching environment.

[0036] The material for which etching is limited by a surface reactionlayer is not limited to any category of materials. In one embodiment,copper is the material that is surface reaction layer limited. Inanother embodiment, a semiconductor is the material that is surfacereaction layer limited. For example, the material is indium phosphide inone embodiment.

[0037] Yet another embodiment of the present invention is a method forreducing a surface reaction layer formed in a low-temperature etchingenvironment. Referring to process 600 of FIG. 6, block 610 comprisesintroducing an inert gas into the low-temperature etching environment.

[0038] Block 620 comprises introducing a halogen into thelow-temperature etching environment. The inert gas and the halogen areintroduced in a ratio that inhibit the formation of an etch-limitingsurface reaction layer during etching in the low-temperature etchingenvironment. The temperature of the low-temperature environment is belowthat which would melt a photoresist material that has not been treatedto increase its glass-reflow temperature.

[0039] While the present invention has been described in particularembodiments, it should be appreciated that the present invention shouldnot be construed as limited by such embodiments, but rather construedaccording to the below claims.

We claim:
 1. A low-temperature etching environment, comprising: ahalogen; and an inert gas; wherein said halogen and said inert gas areprovided in a ratio that does not induce the formation of anetch-limiting surface reaction layer during etching in saidlow-temperature etching environment.
 2. The low-temperature etchingenvironment of claim 1, wherein the temperature of a surface of amaterial being etched in said low-temperature etching environment belowthat which would melt a photoresist material that has not been treatedto increase its glass-reflow temperature.
 3. The low-temperature etchingenvironment of claim 1, wherein said halogen is selected from fluorine,chlorine, bromine, iodine and compounds comprising fluorine, chlorine,bromine, or iodine.
 4. The low-temperature etching environment of claim1, wherein said inert gas is selected from helium, neon, argon, krypton,and xenon.
 5. The low-temperature etching environment of claim 1,wherein said halogen comprises chlorine.
 6. The low-temperature etchingenvironment of claim 1, wherein said inert gas comprises argon.
 7. Thelow-temperature etching environment of claim 1, wherein said ratio ofsaid halogen and said inert gas is achieved by a flow rate of saidhalogen less than 20 percent of the combined flow rate of said halogenand said inert gas.
 8. A method of etching a surface reaction layerlimited material, the method comprising: a) receiving said surfacereaction layer limited material in a low-temperature etching environmentcomprising: a halogen and an inert gas in a ratio that does not inducethe formation of an etch-limiting surface reaction layer during etchingin said low-temperature etching environment; and b) etching said surfacereaction layer limited material within said low-temperature etchingenvironment.
 9. The method of claim 8, wherein said surface reactionlayer limited material comprises a semiconductor.
 10. The method ofclaim 8, wherein said surface reaction layer limited material comprisesindium.
 11. The method of claim 8, wherein said surface reaction layerlimited material comprises copper.
 12. The method of claim 8, whereinsaid halogen comprises chlorine.
 13. The method of claim 8, wherein saidinert gas comprises argon.
 14. The method of claim 8, wherein thetemperature of said low-temperature etching environment is below thatwhich would melt a photoresist material that has not been treated toincrease its glass-reflow temperature.
 15. A method for inhibitingformation of a surface reaction layer formed in a low-temperatureetching environment, said method comprising: introducing an inert gasinto said low-temperature etching environment; introducing a halogeninto said low-temperature etching environment in a ratio to said inertgas that does not induce the formation of an etch-limiting surfacereaction layer during etching in said low-temperature etchingenvironment.
 16. The method of claim 15, wherein the temperature of saidlow-temperature etching environment is below that which would melt aphotoresist material that has not been treated to increase itsglass-reflow temperature.
 17. The method of claim 15, wherein saidhalogen is selected from fluorine, chlorine, bromine, iodine andcompounds comprising fluorine, chlorine, bromine, or iodine.
 18. Themethod of claim 15, wherein said inert gas is selected from helium,neon, argon, krypton, and xenon.
 19. The method of claim 15, whereinsaid halogen comprises chlorine.
 20. The method of claim 15, whereinsaid inert gas comprises argon.
 21. The method of claim 15, wherein saidratio of said halogen to said inert gas is achieved by a flow rate ofsaid halogen less than 20 percent of the combined flow rate of saidhalogen and said inert gas.
 22. The method of claim 15, wherein saidlow-temperature etching environment comprises plasma etching ofsufficient ion density to physically minimize the surface reaction layeras it is being formed.